Optical fiber transmission line

ABSTRACT

An optical fiber transmission line capable of suppressing the generation efficiency of four wave mixing in transmitting wavelength multiplex signal light. The transmission line employs a single mode optical fiber in which a zero dispersion wavelength is varied in the longitudinal direction of the optical fiber more largely than manufacturing variations in manufacturing conditions of the optical fiber. The use of this optical fiber as the transmission line suppresses the generation efficiency of four wave mixing in transmitting the wavelength multiplex signal light to allow the transmission of the wavelength multiplex signal light with a reduced crosstalk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber transmission line andan optical fiber communication system using the optical fibertransmission line.

2. Description of the Related Art

In recent years, a transmission system capable of performing directamplification of light has been studied owing to the advent of an erbiumdoped optical fiber amplifier. In particular, the erbium doped opticalfiber amplifier has a wide gain wavelength band and therefore allowswavelength division multiplexing (WDM) transmission and collectiveamplification and relay of each wavelength. However, the entry of awavelength multiplex signal having a high light power into an opticalfiber causes a new problem such that a nonlinear optical effect in theoptical fiber becomes remarkable.

An optical fiber now in use is formed of a silica-based material, whichis essentially very small in nonlinearity. However, since a light waveis confined in a microscopic region of about 10 μm in diameter, a powerdensity becomes very high, and various nonlinear interactions may ariseremarkably because of a very large length of interaction between thelight wave and the material. Accordingly, the nonlinear optical effectcauses a deterioration of transmission characteristics in WDMtransmission. The nonlinear optical effect of an optical fiber having anadverse effect on WDM includes stimulated Brillouin scattering, mutualphase modulation, Raman scattering, and four wave (photon) mixing.

According to the literature "IEEE J. Lightwave Technol., vol. 6, no. 11,pp. 1750-1769", the four wave mixing (FWM) of the above-mentionednonlinear optical effect gives the severest conditions to the design ofa communication system. That is, light frequency mixing between signallight waves due to FWM causes generation of new FWM waves, which act asa crosstalk with the original signal light waves to deteriorate thetransmission characteristics. The generation efficiency of FWM isdecided by a quantity Δβ of phase mismatch between light waves, and Δβis dependent upon the wavelength space between light waves and thedispersion of an optical fiber. Therefore, in the case of using as thetransmission line a dispersion shifted fiber such that a zero dispersionregion of the fiber is shifted to a 1.5 μm band where a transmissionloss is minimized, the influence of the FWM becomes remarkable.

As an example, FIG. 10 shows a difference in the influence of FWMbetween two kinds of optical fibers as shown in the literature "IEEE J.Lightwave Technol., vol. 10, no. 3, pp. 361-366". In FIG. 10, the solidline shows a usual fiber, and the broken line shows a dispersion shiftedfiber. As the dispersion shifted fiber is largely affected by crosstalk,it is necessary to reduce an input power into the optical fiber, so thatthe transmission characteristics are largely limited. The results ofstudy mentioned in the above literature are those obtained with theassumption that the characteristics of the optical fiber are ideal,i.e., a zero dispersion wavelength is constant in the longitudinaldirection of the fiber. Actually, however, it is considered that thereis a change in dispersion value in the longitudinal direction of theoptical fiber due to variations in manufacturing conditions of theoptical fiber.

Examples of measurement of the distribution of generation efficiency ofFWM in an actual dispersion shifted fiber are shown in FIGS. 11 and 12.FIG. 11 shows the relation between a signal light wavelength and an FWMgeneration efficiency in the case where the fiber length is 1.1 km, andFIG. 12 shows the same relation in the case where the fiber length is 23km. In each of FIGS. 11 and 12, the broken line shows a calculated valueand the solid line shows an experimental value. When the optical fiberis short as shown in FIG. 11, the calculated value and the experimentalvalue are in good agreement with each other, and the generation of FWMis observed at a specific wavelength. To the contrary, when the opticalfiber is long as shown in FIG. 12, the steep peak predicted bycalculation is not measured in actual, but the FWM wave measured isdistributed in a wide range of wavelength. It is understood that thisresult is due to the variations in dispersion value in the longitudinaldirection of the optical fiber.

FIG. 13 shows the result of measurement of variations in zero dispersionwavelength (wavelength where the dispersion value becomes zero) in thelongitudinal direction of the optical fiber. This result is one obtainedby using the optical fiber corresponding to that shown in FIG. 12, andit is understood that a variation of about 3.5 nm is observed. A maximuminclination is 1.1 nm/km. If the variations in zero dispersionwavelength in the longitudinal direction of the optical fiber are alwayspresent, the generation efficiency of FWM is widely distributed, but thegeneration efficiency of FWM at a specific wavelength is reduced.

As an example, the literature "Electronic Information and CommunicationSociety, Autumn Great Meeting B-660, 1992" has proposed a method ofconstructing a transmission line by alternately connecting a fiberhaving a positive dispersion value and a fiber having a negativedispersion value and making the total quantity of dispersion zero asshown in FIG. 14. This literature has reported that such a constructionof the transmission line has improved the transmission characteristics.A primary object of the transmission line shown in this literature is toreduce the crosstalk due to the interaction between noise and signallight from a low-power light amplifier in the case of transmitting aone-wave signal over a long distance.

In general, a noise component is enough smaller than a signal lightpower, and the generation of FWM between noise and signal light becomesa problem in the transmission over the distances of hundreds ofkilometers. Accordingly, it is sufficient to alternate the positivedispersion value and the negative dispersion value at the intervals oftens of kilometers, and it is unnecessary to consider the distributionof the zero dispersion wavelength in each section in the longitudinaldirection of the fiber. For example, there is no harm in makingcompletely uniform the zero dispersion wavelength in each section of thefiber.

In contrast, it is considered that a WDM transmission system maygenerate FWM between high-power signal lights. If a zero dispersionwavelength and a signal light wavelength are very close to each othereven over a short distance, the generation efficiency of FWM is rapidlyincreased. For example, if the zero dispersion wavelength is constantand the signal light wavelength is the same as or close to this evenover a short distance of 1 to 2 km, a crosstalk having an influence onthe transmission characteristics is generated. Accordingly, theabove-mentioned method is not effective in suppressing the crosstalkhaving an influence on the transmission characteristics.

Further, it is necessary to select a specific fiber having suitablecharacteristics from many fibers, which causes a problem in practicaluse. Further, according to the result of measurement on the actualfibers, it cannot be considered that the zero dispersion wavelength ineach span is uniform, and it is essentially difficult to construct thetransmission line by combining the fibers having positive or negativedispersion values and making the total dispersion amount zero. Thedispersion shifted fiber at present has variations difficult to controldue to manufacturing conditions. For example, the literature "IEEE J.Lightwave Technol., vol. 8, no. 10, pp. 1476-1481" has reported 0.03 μmas a measurement value of variations in core diameter.

Further, it is anticipated that a difference Δn in specific refractiveindex between a core and a clad is also varied according tomanufacturing conditions. This difference causes variations in zerodispersion wavelength in the fiber. In the case of WDM transmission bythe use of such a wavelength, the amount of crosstalk due to FWM becomessmaller than that in the estimation that the zero dispersion wavelengthin the fiber is theoretically uniform in the longitudinal direction ofthe fiber. However, if the distribution of variations in zero dispersionwavelength follows a normal distribution, there is a possibility ofproduction of fibers excluding the variations in the longitudinaldirection with a given probability. In the case where a wavelengthmultiplex signal is transmitted through a transmission line constructedby randomly selecting these fibers, there is a possibility that FWM maybe largely generated.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalfiber transmission line which can suppress the generation efficiency ofFWM in transmitting a wavelength multiplex signal.

It is another object of the present invention to provide an opticalfiber communication system which can suppress the generation efficiencyof FWM to transmit a wavelength multiplex signal with a reducedcrosstalk.

In accordance with an aspect of the present invention, there is providedan optical fiber transmission line having a single mode optical fiber inwhich a zero dispersion wavelength is varied in a longitudinal directionof said optical fiber more largely than manufacturing variations inmanufacturing conditions of said optical fiber. Preferably, thevariations in the zero dispersion wavelength intentionally given areperiodic, and a dispersion value is continuously changed in thelongitudinal direction of the fiber. More preferably, the average of thevariations in the zero dispersion wavelength intentionally given is madezero.

It is effective that the signal light wavelength and the zero dispersionwavelength are adjusted not to coincide with each other in suppressingFWM. Therefore, it is necessary that the range of variations and theperiod of variations in the zero dispersion wavelength are made as wideas possible within such a range that a large waveform distortion of thesignal light does not occur. It is also necessary that the variationshave a suitable inclination in the longitudinal direction so as not tomake the zero dispersion wavelength constant.

For example, the lengthwise distribution of the variations in the zerodispersion wavelength intentionally given preferably has a triangularwave shape, a saw-toothed wave shape, a trigonometric function shape, ora random shape. The distribution of the variations having a rectangularwave shape is unsuitable because a dispersion value over a certaindistance is constant. Further, in order to obtain variations cancelingthe influence of the manufacturing variations originally present andbecoming greater than the manufacturing variations, the variations inthe zero dispersion wavelength must be made greater than those due tothe uncontrollable manufacturing variations.

In accordance with another aspect of the present invention, there isprovided an optical fiber communication system for transmittingwavelength multiplex signal light through an optical fiber transmissionline, comprising a single mode optical fiber in which a zero dispersionwavelenght is varied in a longitudinal direction of said optical fibermore largely than manufacturing variations in manufacturing conditionsof said optical fiber; an optical transmitter connected to one end ofsaid single mode optical fiber, for transmitting said wavelengthmultiplex signal light; and an optical receiver connected to the otherend of said single mode optical fiber.

The use of the optical fiber positively varied in its zero dispersionwavelength as the transmission line allows the zero dispersionwavelength not to coincide with a signal light wavelength, therebysuppressing the generation efficiency of FWM. Accordingly, the crosstalkdue to the generation of FWM can be effectively suppressed to allow thetransmission of the wavelength multiplex signal with less deteriorationin transmission characteristics.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the range of variationsin zero dispersion wavelength and the generation efficiency of FWM;

FIG. 2 is a graph showing a distribution model of variations in zerodispersion wavelength;

FIG. 3 is a graph showing the generation efficiency of FWM in the casewhere an average zero dispersion wavelength is shifted from a signallight wavelength;

FIG. 4 is a schematic view showing a method of fabricating a preform fora single mode fiber;

FIG. 5 is a graph showing an example of a perform forming method;

FIGS. 6A and 6B are graphs showing the relation between a preformforming speed and a preform core diameter;

FIG. 7 is a schematic view showing an optical fiber drawing device;

FIG. 8 is a schematic view showing an optical fiber communication systemaccording to a preferred embodiment of the present invention;

FIG. 9 is a schematic view showing an optical fiber communication systemaccording to another preferred embodiment of the present invention;

FIG. 10 is a graph showing the relation between an input power perchannel and a power penalty;

FIG. 11 is a graph showing the relation between a signal lightwavelength and the generation efficiency of FWM in the case where thefiber length is 1.1 km;

FIG. 12 is a graph showing the relation between a signal lightwavelength and the generation efficiency of FWM in the case where thefiber length is 23 km;

FIG. 13 is a graph showing variations in zero dispersion wavelength; and

FIG. 14 is a graph showing an example of an optical fiber transmissionline in the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described indetail with reference to the drawings.

FIG. 1 shows the result of calculation of the generation efficiency ofFWM after transmitting signal light of two waves through a dispersionshifted fiber (DSF) having a length of 7 km. In FIG. 1, the horizontalaxis represents a half width of the range of variations in zerodispersion wavelength, and the period of variations and the wavelengthspace between the signal waves are used as parameters. The broken lines(a) in FIG. 1 correspond to the period of variations in zero dispersionwavelength shown in the graph (a) in FIG. 2, and the solid lines (b) inFIG. 1 correspond to the period of variations in zero dispersionwavelength shown in the graph (b) in FIG. 2. Further, the wavelengthspace Δλ between the two waves is set to 2 nm and 10 nm.

In FIG. 1, the dots show the generation efficiency of FWM in the casewhere the zero dispersion wavelength in the fiber is uniform and thesignal light wavelength is coincident with the zero dispersionwavelength. As apparent from FIG. 1, the larger the variations in zerodispersion wavelength, the smaller the generation efficiency of FWM.Further, the larger the period of variations, the more effective thedecrease in the generation efficiency of FWM.

Referring to FIG. 3, there is shown the result of calculation of thegeneration efficiency of FWM after transmitting signal light through adispersion shifted fiber having a length of 7 km in the case where anaverage zero dispersion wavelength is shifted from a signal lightwavelength. In FIG. 3, the solid line corresponds to a conventionalfiber in which no variations are given to the zero dispersionwavelength, and the broken line corresponds to the fiber of the presentinvention in which variations are positively given to the zerodispersion wavelength. An input power of 10 dBm is applied to eachfiber.

As apparent from FIG. 3, in the fiber having the variations in zerodispersion wavelength, a wavelength region of generation of FWM iswidened, but the absolute value of the generation efficiency is reduced.In the case of designing a crosstalk amount to -30 dB or less, forexample, this design value cannot be satisfied in the prior art fiberwherein the zero dispersion wavelength is constant and the signal lightwavelength is coincident with the zero dispersion wavelength. However,according to the fiber of the present invention wherein the zerodispersion wavelength is intentionally varied, a desired design valuecan be satisfied no matter how the average zero dispersion wavelengthand the signal light wavelength are related with each other.

As apparent from the above description, it is effective that the signallight wavelength and the zero dispersion wavelength are adjusted so asto differ from each other in suppressing FWM. Therefore, it is necessarythat the range of variations and the period of variations in the zerodispersion wavelength are made as wide as possible. It is also necessarythat the zero dispersion wavelength is continuously changed in thelongitudinal direction of the fiber so as to avoid a constant zerodispersion wavelength. While the range of variations in zero dispersionwavelength is desirably made as wide as possible to suppress thegeneration efficiency of FWM as mentioned above, it is necessary tosuppress the range of variations in zero dispersion wavelength to about30 nm or less from the viewpoint of suppression of waveform distortion.For example, the fiber lengthwise distribution of variations in zerodispersion wavelength intentionally given preferably has a triangularwave shape, a saw-toothed wave shape, a trigonometric function shape, ora random shape.

For example, a single mode optical fiber continuously changing inpositive dispersion value with respect to the signal light to betransmitted and a single mode optical fiber continuously changing innegative dispersion value are connected together so as to nullify thetotal dispersion, thus constructing an optical fiber transmission line.Accordingly, the waveform distortion due to the dispersion can becanceled to suppress the crosstalk due to the generation of FWM. Thevariations in zero dispersion wavelength may be realized by changing adifference in specific refractive index between a core and a clad of theoptical fiber in the longitudinal direction thereof. Alternatively, thevariations in zero dispersion wavelength may be realized by changing acore diameter of the optical fiber in the longitudinal directionthereof.

Referring to FIG. 4, there is schematically shown a method ofmanufacturing a preform for a single mode fiber. The method shown is aprocess of axial vapor deposition, in which a preform 12 formed israised in the direction of an arrow A. A stock gal is blown from a coreburner 2 against the bottom of a rotating rod to form a porous preform 4as a core portion. The stock gas contains H₂, O₂, SiCl₄, and GeO₂ l forexample. The addition of GeO₂ to the stock gas allows an increase inrefractive index of the porous preform 4.

A stock gas is next blown from clad burners 6 and 8 against the porouspreform 4 to form a porous preform 10 as a clad portion. The stock gascontains H₂, O₂, and SiCl₄, for example. Examples of a dopant to beadded to the core portion 4 may include GeO₂, P₂ O₅, and Al₂ O₃.Further, examples of a dopant to be added to the clad portion 10 mayinclude B₂ O₃ and F capable of decreasing a refractive index.

In forming the preform 12 shown in FIG. 4, the concentration of thedopant to be added to the core portion 4 or the clad portion 10 ischanged as shown in FIG. 5 to thereby vary a difference in specificrefractive index between the core portion 4 and the clad portion 10 ofthe preform 12 in the longitudinal direction. Then, the preform 12 isdrawn to form an optical fiber, thereby lengthwise varying thedifference in specific refractive index between a core and a clad of theoptical fiber.

Referring to FIGS. 6A and 6B, there is shown the relation between apreform forming speed and a core diameter in the preform. As apparentfrom FIGS. 6A and 6B, the core diameter in the preform is changed bychanging the preform forming speed with the dopant concentration keptconstant. Accordingly, the preform thus formed is drawn to form anoptical fiber, thereby changing the core diameter of the optical fiberin the longitudinal direction thereof. As a result, a zero dispersionwavelength can be varied in the longitudinal direction of the opticalfiber.

Referring to FIG. 7, there is schematically shown an optical fiberdrawing device. The preform 12 is mounted on a precision mount 14, andis gradually fed into a drawing oven 16 at a suitable speed. In thedrawing oven 16, the p reform 12 is partially heated to 2000° C. orhigher. The preform 12 softened by the heat is thinned, and this iscontinuously drawn to form an optical fiber. Du ring this drawingoperation, the diameter of the optical fiber is continuously monitoredby a measuring instrument 18 such as a laser micrometer.

The optical fiber drawn from the preform 12 is immediately covered withplastic by a primary covering device 20. The plastic cover serves toprotect the fiber from moisture and abrasion. A suitable material forthe plastic cover may be a silicone resin, epoxy resin, etc. The opticalfiber thus covered with the plastic by the primary covering device 20 isfed through a capstan 24 and a dancer roller 26 to a drum 28, and iswound around the drum 28. A controller 22 is connected to the measuringinstrument 18, the capstan 24, the dancer roller 26, and the drum 28 tocontrol these members.

That is, the controller 22 controls these members so as to vary thedrawing speed of the optical fiber to be drawn from the preform 12.Accordingly, the core diameter of the optical fiber can be varied in thelongitudinals direction of the fiber. Alternatively, the temperature inthe drawing oven 16 may be varied with the drawing speed kept constant,thereby varying the core diameter of the optical fiber.

An optical fiber communication system employing the optical fibertransmission line of the present invention will now be described withreference to FIG. 8. The transmission line is constructed of a singlemode optical fiber 30 in which a zero dispersion wavelength ispositively varied in the longitudinal direction. An optical transmitter32 for transmitting wavelength multiplex signal light is connected toone end of the optical fiber 30, and an optical receiver 34 is connectedto the other end of the optical fiber 30. Since the generationefficiency of FWM in the optical fiber 30 is low, the wavelengthmultiplex signal light transmitted from the optical transmitter 32 isreceived by the optical receiver 34 without the occurrence of crosstalk.

Referring to FIG. 9, there is schematically shown another preferredembodiment of the optical fiber communication system. In this preferredembodiment, a single mode optical fiber 30 in which a zero dispersionwavelength is intentionally varied is used as a transmitting sideportion of the transmission line where light power is large, and a usualsingle mode optical fiber 36 in which a zero dispersion wavelength isnot varied is used as the other portion of the transmission line, thusperforming the transmission of wavelength multiplex signal light.

According to the present invention, the use of an optical fiber in whichthe factor of generation of FWM in the fiber is controlled in the stageof manufacture of the fiber can eliminate the need for control of adispersion value in laying the fiber to allow the transmission ofwavelength multiplex signal light with a reduced crosstalk.

What is claimed is:
 1. An optical fiber transmission line comprising:asingle mode optical fiber in which a zero dispersion wavelength isvaried in a longitudinal direction of said optical fiber more largelythan manufacturing variations in manufacturing conditions of saidoptical fiber: said single mode optical fiber being constructed byalternately connecting an optical fiber continuously changing indispersion value with respect to signal light to be transmitted andhaving a positive average dispersion value and an optical fibercontinuously changing in dispersion value and having a negative averagedispersion value.
 2. An optical fiber transmission line comprising:asingle mode optical fiber in which a zero dispersion wavelength isvaried in a longitudinal direction of said optical fiber more largelythan manufacturing variations in manufacturing conditions of saidoptical fiber; wherein variations in said zero dispersion wavelength areperiodic, and a dispersion value is continuously changed in thelongitudinal direction of said optical fiber.
 3. An optical fibertransmission line according to claim 2, wherein a lengthwisedistribution of the variations in said zero dispersion wavelength has ashape selected from the group consisting of a triangular wave shape, asaw-toothed wave shape, a trigonometric function shape, and a randomshape.
 4. An optical fiber transmission line according to claim 1,wherein said optical fiber having the positive average dispersion valueand said optical fiber having the negative average dispersion value areconnected together so that total dispersion becomes zero.
 5. An opticalfiber transmission line according to claim 3, wherein an average of thevariations in said zero dispersion wavelength is made zero.
 6. Anoptical fiber communication system for transmitting wavelength divisionmultiplexed signal light through an optical fiber transmission line,comprising:a single mode optical fiber in which a zero dispersionwavelength is varied in a longitudinal direction of said optical fibermore largely than manufacturing variations in manufacturing conditionsof said optical fiber, said single mode optical fiber being constructedby alternately connecting an optical fiber continuously changing indispersion value with respect to signal light to be transmitted andhaving a positive average dispersion value and an optical fibercontinuously changing in dispersion value and having a negative averagedispersion value; an optical transmitter connected to one end of saidsingle mode optical fiber, for transmitting said wavelength divisionmultiplexed signal light; and an optical receiver connected to the otherend of said single mode optical fiber.
 7. An optical fiber communicationsystem for transmitting wavelength division multiplexed signal lightthrough an optical fiber transmission line, comprising:a first singlemode optical fiber in which a zero dispersion wavelength is varied in alongitudinal direction of said optical fiber more largely thanmanufacturing variations in manufacturing conditions of said opticalfiber, said single mode optical fiber being constructed by alternatelyconnecting an optical fiber continuously changing in dispersion valuewith respect to signal light to be transmitted and having a positiveaverage dispersion value and an optical fiber continuously changing indispersion value and having a negative average dispersion value; asecond single mode optical fiber having one end connected to one end ofsaid first single mode optical fiber, said second single mode opticalfiber having no intentional variations in zero dispersion wavelength; anoptical transmitter connected to the other end of said first single modeoptical fiber, for transmitting said wavelength division multiplexedsignal light; and an optical receiver connected to the other end of saidsecond single mode optical fiber.
 8. An optical fiber communicationsystem according to claim 6, wherein variations in said zero dispersionwavelength are periodic, and a dispersion value is continually changedin the longitudinal direction of said optical fiber.
 9. An optical fibercommunication system according to claim 7, wherein variations in saidzero dispersion wavelength are periodic, and a dispersion value iscontinually changed in the longitudinal direction of said optical fiber.